1. Field of the Invention
The present invention is directed to a nuclear power plant core power distribution mapping and display system and, more particularly, to a system that uses a limited number of fixed incore detectors and core exit thermocouples to synthesize a full core power distribution.
2. Description of the Related Art
The use of many strings of fixed incore neutron or gamma ray sensitive detectors to continuously monitor the three dimensional nuclear power distribution in a nuclear power reactor core is common. In applying the concept of continuous, online power distribution monitoring with the aid of fixed incore detectors, it is normal practice to install one string, containing four to seven individual detectors located axiallly over the span of the core height, for every three to four fuel assemblies in the core. One can expect to find fifty to sixty such strings of detectors in a large (1000 MWe) nuclear power reactor core. Such an arrangement of detector strings is called a full complement of fixed incore detectors. With a suitably large number of strings of detectors installed and with the conventional digital computer software and hardware presently available, it is possible to generate and update on a one to two minute cycle a full three dimensional core power distribution representation with sufficient accuracy and precision to allow system designers to reduce the uncertainty penalties imposed on the operating power safety margins incorporated in their designs. This then also allows reduction in operating constraints imposed to account for uncertainties in power distribution and provides the operators of these full string complement facilities with additional operating space, that is, provides a larger range of safe operation.
The additional operating space made available by the continuous, online use of installed fixed incore detectors can be utilized to improve plant availability and responsiveness to load changes and reduce the costs of replacement power. However, the costs of the large number of fixed incore detector strings and the supporting electronics for signal sampling and transmission to the computing system are relatively large and, in a nominally base load operating environment, it is not clear that the benefits obtainable from the described system outweigh the costs by any economically significant amount.
For this reason, some manufacturers do not supply a full complement of fixed incore detectors as standard equipment, but rather provide a movable incore detector system by which miniature neutron detectors are mechanically moved through special instrumentation thimbles in selected fuel assemblies. The traces obtained by recording detector output current as the detectors are passed through the entire height of the core in each of the many such instrumentation thimbles can then be used to synthesize the three dimensional core power distribution that existed at the time of mapping and to demonstrate (after the fact) that the core power distribution was safely within the prescribed operating limits.
The costs associated with a movable incore detector system are lower than those of a full complement fixed incore detector system by a factor of perhaps two to three and the accuracy and precision of the synthesized core power distributions are at least as high as is obtainable with fixed incore detector systems. However, the mechanical components of the movable incore detector system are subject to wear and so use of the system is necessarily limited. Continuous monitoring of the core power distribution with a movable detector system over the life span of a nuclear power plant is at best impractical and probably not achievable without periodic replacement of the mechanical and neutron sensing components which increases system cost.
What is desired is a means for continuously synthesizing the core three dimensional power distribution with degrees of accuracy and precision comparable to those obtainable with a full complement fixed incore detector system or a movable incore detector system at a plant lifetime cost substantially less than that of a full incore detector system and at only a modest increment over the initial cost of a movable incore detector system. The system should be such that the use of an already installed movable incore detector system can be reduced, on the average, during the remainder of plant life span to reduce maintenance costs and radiation exposure of maintenance personnel.
U.S. Pat. No. 4,774,049 describes an approach to continuous full core power distribution synthesis based on: (1) the use of signals from excore neutron detectors, supplemented as necessary by signals from selected core exit thermocouples, to synthesize peripheral axial power distributions; (2) the use of a library of X-Y multiplier values to propagate the synthesized axial power distributions transversely to regions of the core other than those in the field of vision of the excore neutron detectors; and (3) the use of the full complement of core exit thermocouples to adjust, in the X-Y sense, the constructed three dimensional core power distribution to account for existing X-Y tilts in the actual core power distribution.
Even though the methodology of U.S. Pat. No. 4,774,049 will yield synthesized core power distributions of sufficient accuracy and precision to meet power distribution surveillance requirements and to obtain for the operator a relaxation of existing, restrictive technical operating specifications, it will not achieve the degree of accuracy and precision obtainable with either a full fixed incore detector system or a movable detector system. If one or more strings of fixed incore detectors are operational in a core being monitored, the signals from the fixed incore detectors could be used in the same way as the signals from the excore detectors, i.e., to synthesize local axial power distributions.
Another conventional method of core mapping using the movable incore detector system is called "quarter core" flux mapping. "Quarter core" flux mapping with a movable detector system is appropriate during periods of transient reactor operation where good accuracy and precision in measurements of the core average axial power distribution are needed and when comparable accuracy and precision in the X-Y components of the core power distribution can be sacrificed to some degree. A typical application of quarter core flux mapping is in the incore/excore axial offset calibration procedure conducted periodically in all Westinghouse reactors. A description of this type of mapping is provided in Westinghouse Report WCAP 8648 entitled "Excore Detector Recalibration Using Quarter Core Flux Maps" by R. A. Kerr 1976 and can be found in the public document room of Nuclear Regulatory Commission. Making a quarter core flux map includes obtaining detector response traces at a limited number of X-Y locations in the core. Typically two or three passes along the Z axis are made with each movable detector drive, yielding in a normal four loop core no more than twelve or eighteen traces. Each normalized trace in each of the core quadrants is translated to symmetric X-Y locations in each of the other three quadrants. The local axial power distributions derived from the actual and translated traces are interpolated and/or extrapolated to produce a full three dimensional core power distribution.
The deficiencies of quarter core flux mapping, as compared to full core flux mapping, in which all accessible thimbles are indeed accessed by the movable detector system and detector responses at all available X-Y locations are recorded, are two fold: (1) the data available for normalization of the respective detector response traces are incomplete, since a total of six passes by each of the movable detector drives (in a four loop plant) is necessary to obtain direct intercalibration of all detectors in a mutually common thimble; and (2) information regarding true X-Y or azimuthal tilts in the actual core power distribution is lost in the trace transfer process. Regardless of reality, the synthesized core power distribution will show azimuthal symmetry from quadrant to quadrant.